A. Field of the Invention
The present invention relates to apparatuses for monitoring intracranial pressure in diagnostic and post-operative situations, and more particularly, to an intracranial pressure transensor which is entirely implantable within a living body and which includes a resonant circuit that varies in frequency in a direct relationship with variations in pressure.
B. Description of the Prior Art
The need for monitoring intracranial pressure in patients having certain physiological conditions has been long recognized. These conditions include hydrocephalic conditions, conditions where patients have undergone neurosurgery, and other critical situations where individuals are subject to brain swelling, edema, obstruction of cerebral spinal fluid pathways, or intracranial space occupying lesions. Convenient and accurate monitoring of the intracranial pressure in these situations frequently allows institution of correctional emergency procedures when intracranial pressure rises or falls to dangerous levels.
Present methods for monitoring intracranial pressure include implantation of a pressure transducer which requires a portion thereof, such as a wire or coupling to pass through the skull and scalp of the subject in which the device is implanted. Such a technique is largely unsatisfactory as danger of infection, patient discomfort, reduced patient mobility, and the use of surgical procedures to remove the apparatus when it is no longer needed are inherent.
Intracranial pressure monitoring apparatuses which are entirely implantable and employ variable frequency tuned circuits whose frequencies vary in relation to pressure are also known. These apparatuses mechanically translate a change in pressure into a change in reactance of a reactive component of the L/C circuit employed so that the frequency thereof shifts. This relationship is embodied in apparatuses presently known so that an increase in pressure will cause either an increase in capacitance or inductance in the tuned circuit thereby causing a decrease in the resonant frequency of the circuit. Typical of apparatuses employing such a relationship are those disclosed in U.S. Pat. No. 3,958,558 issued to R. R. Dunphy et al and U.S. Pat. No. 4,026,276 issued to J. G. Chubbuck on May 31, 1977, said latter patent being assigned to the assignee of the present invention. A decrease in frequency as a result of an increase in pressure as monitored by the above-noted intracranial pressure monitors, when the variable reactive component is a capacitor, leads to a situation where greatest read-out sensitivity is at the high end of the pressure range which can be monitored, since that is where a change in pressure produces the greatest change in capacitive reactance and therefore produces greatest frequency shift. However, it is desirable to have maximum sensitivity at pressures closest to normal since this is when corrective action is first indicated.
In addition, presently known apparatuses of the character described above which employ variable capacitors and fixed inductors are disclosed as approaching maximum capacitance as pressure increases. As a result, highly elevated cranial pressures, in certain circumstances, can exceed the maximum limits of the variable capacitor employed, likely leading to shorting of the capacitor and therefore temporary inoperation of the monitoring process. Another feature of presently known intracranial pressure monitors that employ resonant circuits is the relationship between the variable capacitance of the resonant circuit and the stray capacitance substantially attributable to conductive body fluids in which the monitor is implanted. Minimum sensitivity as noted above, as well as maximum frequency shift due to implantation into a conductive fluid is inherent in presently known apparatuses since the ratio of variable capacitance of the resonant circuit to the stray capacitance provided by the conductive body fluids is at a minimum at pressures close to normal.
An additional characteristic of presently known intracranial pressure monitors which employ L/C resonant circuits is difficulty in calibration. Typically, these apparatuses are small in size and tolerances of much less than a thousandth of an inch are not uncommon to insure accurate calibration of the mechanical structures associated with the reactive element of the resonant circuit which varies in response to changes in pressure. Presently known apparatuses rely upon the dimensional stability of the housing which houses the resonant circuit and the reactive component which is varied to insure calibration of that component. As a result, the housing itself, which might be manufactured and assembled to lower tolerances, must be made to conform to otherwise unnecessarily exacting standards. Trying to attain these standards in a housing is difficult and costly.
The present invention provides several significant advances over the prior art by providing an intracranial pressure implant which has its greatest read-out sensitivity at pressure levels closest to normal, which is subject to inconsequential frequency shifts due to implantation, which is not subject to inoperation due to high pressure levels experienced during use, and which insures dimensional stability between the mechanical components of the variable reactive element of the inductor capacitor-resonant circuit employed therein, without reliance upon dimensional stability of the housing which encloses the resonant circuit.